In thermally assisted optical/magnetic data storage, information bits are recorded on a layer of a storage medium at elevated temperatures, and the heated area in the storage medium determines the data bit dimension. In one approach, an electromagnetic wave in the form of light is used to heat the storage medium. For recording, it is preferred to have a high light throughput to the storage layer of the medium.
Heat assisted magnetic recording (HAMR) generally refers to the concept of locally heating a recording medium to reduce the coercivity of the recording medium so that the applied magnetic writing field can more easily direct the magnetization of the recording medium during the temporary magnetic softening of the recording medium caused by the heat source. Heat assisted magnetic recording allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature to assure sufficient thermal stability. Heat assisted magnetic recording can be applied to any type of magnetic storage media, including tilted media, longitudinal media, perpendicular media and patterned media.
Heat assisted magnetic recording requires an efficient technique for delivering large amounts of light power to the recording medium confined to spots of, for example, 50 nm or less. Recent designs of HAMR recording heads include a thin film waveguide on an AlTiC slider to guide light to a storage medium for localized heating of the storage medium. To launch light into the waveguide, a grating coupler can be used. Due to the limited size of the slider, the size of the incident beam is only ˜50 μm. At this beam size and with conventional symmetric surface-corrugation grating couplers, coupling efficiency from the incident beam to the waveguide is low (<20%).
To increase coupling efficiency, a highly reflective layer/mask, called a mirror, can be used to reflect the beam transmitted through the grating and waveguide back into the grating region.
There is a need for a waveguide that can provide increased efficiency for coupling an electromagnetic wave into a waveguide.